Olympic gold medalist Sanya Richards-Ross pulls on compression sleeves before a 400-meter race at the World Indoor Athletics Championships in Istanbul in 2012. Martin Meissner/AP hide caption

itoggle caption Martin Meissner/AP

Greg O'Brien gathers his thoughts before a run in 2013. "Running is essential," he says. Michael Strong/Living With Alzheimers hide caption

itoggle caption Michael Strong/Living With Alzheimers

Coss Marte started selling drugs at age 13 and by 19, he says, he was making $2 million a year. After serving a prison sentence, he founded a "prison style" fitness boot camp. Justin Fennert/Courtesy of Behind the Stache hide caption

itoggle caption Justin Fennert/Courtesy of Behind the Stache

Joyce Powell, 80, attends an exercise class at UT-Arlington with her husband, Thomas (right). Powell says she feels more confident in getting around and traveling since taking the classes. Dane Walters/KERA hide caption

itoggle caption Dane Walters/KERA

Varying speed while walking may make the activity much more effective. iStockphoto hide caption

itoggle caption iStockphoto

TK Leif Parsons for NPR hide caption

itoggle caption Leif Parsons for NPR

Dr. Robert Zarr, second from right, leads a hike through a park in Washington, D.C. Diana Bowen/National Park Service hide caption

itoggle caption Diana Bowen/National Park Service

An ad for Vampt's Lean Machine "recovery ale," which will be marketed as a sports drink later this year, if funding allows. Researchers say drinking beer after working out has some advantages, but there are big caveats. Courtesy of VAMPT hide caption

itoggle caption Courtesy of VAMPT

The CDC would be happy with these guys, who were playing in Birmingham, Ala., in July 2013. Teenage boys say basketball is their favorite activity. Mark Almond/AL.COM /Landov hide caption

itoggle caption Mark Almond/AL.COM /Landov